The development of systems of earth orbiting satellites which carry on-board radio transmitters capable of transmitting respective satellite orbital position and time with great accuracy has resulted in the further development of signal receiver devices which can utilize signals received from several satellite transmitters to determine accurate latitude and longitude of the receiver and also calculate altitude, speed and heading. For example, a constellation of satellites in orbits of about eleven thousand miles above the earth's surface and having an orbital period of about twelve hours, known as the Global Positioning System (GPS), has spawned the development of GPS receivers which can determine latitude and longitude to within about thirty meters (typically) of true position on the earth's surface or in an airborne vehicle. The other above-mentioned parameters can also be easily determined by more complex versions of GPS receivers.
Satellite-borne communications systems have also been developed which utilize a series of geosynchronous satellites, which appear to be stationary relative to a given point on the earth's surface and are capable of transmitting and receiving signals between two points spaced apart on the earth's surface or at altitudes less than the altitudes of the orbiting geosynchronous satellites. The geosynchronous satellite constellations are positioned in well-defined equatorial orbits and provide reliable communications. However, the high orbits of these satellites require significant power for transmitting signals to a receiver via one or more of the satellites.
Although, it is possible to provide means for determining the position of an object, such as a vehicle, on the face of the earth or above the earth's surface, using signals from an array of satellites, such as the GPS system, and then relaying this information to a remote station via a geosynchronous satellite communications system, the power requirements for repeatedly transmitting such position tracking information to a user or users desiring to have remote tracking information available, are significant. However, all of the ramifications of such an arrangement have not been contemplated in the development of applications, both commercial and military, for geolocation satellite systems such as the GPS system. Moreover, as described above, remote tracking information regarding the location of a vehicle or other object must be relayed from the vehicle or object being tracked to the party of interest over a separate communications link. Accordingly, there has been a substantial need, in both military and commercial applications, to be able to determine continuously or at will the location of a stationary or moving object on the face of the earth or in a vehicle above the earth and to provide for communications from the vehicle or object to a particular point utilizing one or more satellites in relatively Low Earth Orbits, sometimes known as LEO satellites.
Communication techniques using LEO satellites have been developed wherein a transmitter attempting to transmit a signal to a receiver by way of a LEO satellite is operated repeatedly or continuously in hopes of establishing communication during times when the LEO satellite is "visible" to the transmitter. A LEO satellite may, for example, be in an orbit of only about 150 miles above the earth's surface and therefore be "visible" for only about 5 minutes to 15 minutes. Accordingly, this type of communication also requires significant electrical power to support the repeated or redundant signal transmissions. Moreover, the satellite communication channel or frequency which is handling the redundant transmissions is thus occupied by these repeated transmissions and is otherwise unavailable for transmitting other information from other sources.
Conventional techniques to reduce redundant signal transmissions are not effective when a communications device is non-stationary and the world-wide location is unknown, and such techniques may be detrimental to reliable communications methods. Moreover, although it is known to control signal transmissions from stationary locations on the Earth's surface at specific times or a reduced set of signal transmission times, based on known LEO satellite visibility times, heretofore there have not been any systems or methods developed for communications from a moving or stationary object to a base station via selected ones of several LEO satellites based on the location of the object and ephemeris data for the LEO satellites, respectively.
Accordingly, there has also been a strongly felt need to develop an autonomous system which is capable of determining the location of a particular object, such as a vehicle or person, on the surface of the earth or thereabove (or even at a subsurface location) which is capable of determining the optimum time to transmit a signal or signals to a selected LEO satellite to eliminate the need for repeated redundant transmissions and allow the communications device to be reduced in size by minimizing its power source (such as a battery) capacity. Still further, it has been deemed desirable to provide an autonomous geolocation and message communications device which may utilize LEO satellites for transmission of geolocation and/or other messages and which is physically small enough to be hand-held and carried by a human user, for example. It is to these ends that the present invention has been developed.